To test the thermodynamic stability of an enzymically active derivative (LH1) of hen egg lysozyme, containing three, presumably, native disulfide bonds and carboxymethylated cysteines 6 and 127, three different disulfide bonded forms of LH1 (native, reduced and "scrambled") are exposed to the same concentration of beta-mercaptoethanol (1.5 mM) which permits the disulfide interchange. At the apparent equilibrium state, all three samples show similar ratio (3:2) of native to non-native forms. Thus, the native form is themodynamically favored over the non-native forms. The intermediates, formed during thiol-catalyzed renaturation of the scrambled disulfide bonds, have been trapped by carboxymethylation with 1-14C-iodoacetic acid. Analysis of the labeled tryptic peptides obtained from the renatured forms appears to indicate that equilibrium through the disulfide interchange between the native and non-native isomers containing three disulfide bonds and two sulfhydryl groups is the main event of this renaturation.